The invention relates to the field of signal processors, and, more particularly, to a video signal processor.
Analog video signals are divided into frames and lines. Each frame corresponds to an image and each line corresponds to a horizontal scanning of a television screen. Conventionally, the frames include a number of lines that are greater than the number of lines of the screen. In SECAM, for example, only 525 lines are used for a screen image out of 625 lines transmitted. The lines unused for the transmission of television images are used firstly, for synchronization, and secondly, for data transmission. Among the data transmitted, there are known data elements relating to teletext systems.
Teletext data elements are placed at the beginning of each frame on a few lines. These data elements are structured in numbered pages. Each page represents about 1 k of data elements in text format.
For transmission, the pages are subdivided into several pieces and encoded. Despite the compression of the pages, a single page is transmitted in pieces over several frames. In addition to the page data elements, additional communication and management data elements are transmitted. Depending on the different standards used, which are known as xe2x80x9cbasicxe2x80x9d, xe2x80x9cFLOFxe2x80x9d and xe2x80x9cTOPxe2x80x9d, the processing of the data elements and the management of the pages may be more or less important. The total pages transmitted is commonly hundreds of pages. The full sequence of teletext pages may be transmitted within a few minutes.
In the basic standard, the management data elements are limited to the numbers of each page so that a user can address them. In the FLOF standard, the management data elements associated with each page are the number of the page and other numbers corresponding to other pages which form links enabling the pages to be changed at higher speed for the user. In the TOP standard, the management data elements include a number of the page and of the tables of links with several levels forming a tree structure.
The first television receivers fitted with these systems were unable to function, except under the basic standard, and were able to store only the current page. The teletext system was relatively slow for the user, who had to wait until the page he wanted had been transmitted before being able to view it. In a first improvement, the television receivers were provided with a memory, e.g., an 8 k size memory, to store a few pages. As the television receiver stores the successive pages up to the current page, the user was able to make the pages scroll one after the other without having to systematically wait for the transmission of the desired page, provided that all the pages were read in succession.
The FLOF standard improves the comfort of the user because it enables the television receiver to store subsequent pages, and pages corresponding to the links. Thus, a contents page or a page referring to distant pages enables the user to carry out a page change that is not successive and faster. One problem of the FLOF standard lies firstly, in the waiting period needed when the user makes two successive page skips at high speed, and secondly, in the time needed to return to a contents page when a wrong choice has been made if the contents page has already been overwritten by another one.
The TOP standard enables these problems to be overcome. This is because the management data elements include rising and descending tree structures. The television receiver may then store one or two pages that refer to the current pages, a few following pages, and the pages to which the current page refers. Furthermore, as soon as a page skip is done, the pages to which the current page refers are already known by the television receiver. This makes it possible to anticipate loading of a new page.
The management of teletext systems is presently done in advanced television receivers by specialized circuits which deal only with teletext. These specialized circuits are large-sized and therefore cost a great deal. To integrate the a management of teletext in less sophisticated television receivers, it is necessary to reduce the cost of thee components dedicated to teletext. Present day television receivers include all the microcontroller type circuits using at least one processor. Those skilled in the art have had the idea of integrating a circuit into the microcontroller, with this circuit being dedicated to the extraction of teletext data elements from the video signal.
To reduce the size of the resulting microcontroller, only one circuit performing a rough In extraction of the video signal is integrated. Microcontrollers of this kind are known in the prior art. FIG. 1 gives an exemplary view of the internal diagram of a circuit marketed under the reference ST92R195 by STMicroelectronics S.A. which is the assignee of the present invention. This circuit integrates a rough extraction circuit. The term rough extraction should be understood to mean only the demodulation of the data elements and the storage of the data elements just after demodulation. For further details of the operation of the circuit or its implementation, those skilled in the art may refer to the datasheet for this circuit. This datasheet has been available to the public since November 1997 through STMicroelectronics S.A., and is entitled xe2x80x9cST92R195 8-bits MCU DATASHEET.xe2x80x9d Those skilled in the art may also refer to the literature on other circuits of the same family which are referenced as ST92195, ST92T195 and ST92E195.
Microcontrollers then manage the total teletext system in addition to the total function of the television receiver. It may appear to be a simple matter to program a processor for all these tasks. However, in the field of television receivers, it is extremely costly to use microcontrollers operating at high clock frequencies. A high clock frequency creates an additional noise source which is undesirable. A problem then arises in carrying out both the management of the television receiver and the management of the teletext system with one processor whose processing speed is limited.
FIG. 2 shows timing diagrams firstly, of the rough extraction 201, and secondly, of the tasks of the processor 202 in the case where the teletext is managed at the same level as the management of the television receiver. The timing diagram of the rough extraction 201 shows the instants of extraction 203 which are cyclical with a period T corresponding to the duration of a frame, which is typically 20 ms or 16.6 ms depending on the format of the country implemented therein. The timing diagram of the tasks 202 shows the time-sharing between the different tasks processed by the processor of the microcontroller. The different tasks are herein separated into two types of tasks, namely tasks 204 related to the management of the television receiver and tasks 205 related to the management of the teletext system.
The management of the different tasks 204 as well as the management of the teletext 205 are variable in duration. Consequently, there is a timing drift that may cause a loss of information. FIG. 1 shows a case of a loss where the new data elements extracted replace the data elements previously extracted during the processing of the previously extracted data elements. This results in a loss of data which requires a wait for the lost data elements to be repeated when they are repeated during the next transmission, which may typically be many minutes later. Those skilled in the art realize that it is possible to use an interrupt that corresponds to the beginning of the image, namely, to the end of the rough extraction of the data. An operation of this kind is shown in FIG. 3.
FIG. 3 shows timing. diagrams firstly, of the rough extraction 301, and secondly, of the tasks 302 of the processor. The rough extraction timing diagram 301 shows the instants of extraction 303 which are cyclical with a period T corresponding to the duration of a frame, which it typically 20 ms or 16.6 ms depending on the format of the country implemented therein. The timing diagram of the tasks 302 shows the time-sharing of different tasks processed by the processor of the microcontroller. The different tasks are separated into two types of tasks. Namely tasks related to the management of the television receiver TTV, and those related to the management of the teletext TTXT. Each management task of the teletext TTXT is activated by an end of a vertical blanking interval interrupt or EOVBIT.
A problem may arise when a teletext system is provided in the TOP standard, because the processing of a data packet may last more than the time interval between two data packets. The interrupt EOVBIT is then placed on standby and processed after the processing of the data packet which is under processing. A timing drift may occur and cause a loss of data. This is due to the lengthy processing needed for the management of the tree structure. However, it is also possible that the programs coming from interrupts having higher priority than the teletext system will slow down the processing. This may also prompt a loss of information in the FLOF standard.
An object of the invention is to provide a teletext management method that overcomes the above-mentioned problems. The method separates the processing of data into at least two tasks. A first task deals with the processing of the data elements from the rough extraction, and a second task deals with the rest of the processing. The first task activates the second task by a software interrupt with a lower priority than the end of a vertical blanking interval interrupt in such a way that a next end of a vertical blanking interval interrupt may interrupt the second task to obtain a next first task. Thus, there is no longer any temporal or timing drift that might cause a loss in the data packet from the rough extraction.
Another object of the invention is to provide a method for the acquisition and display of data elements from a video signal received by a television receiver including a processor. The processor receives a first interrupt when the data elements have been extracted from the video signal and stored in a buffer memory. The first interrupt activates a first processing operation for processing the data elements to decode, select and transfer the data elements into a useful memory. The end of the first processing operation produces a second interrupt with a lower priority than the first interrupt. The second interrupt activates a second processing operation for displaying the data elements on a screen and then updating a database used during the first processing operation.
More generally, the invention relates to a method for the management of a television receiver including a microcontroller that includes a processor and peripherals pertaining to the overall function of the television receiver, wherein the processor carries out the management of all the peripherals of the television receiver. The method preferably further includes acquiring and displaying data elements from the video signal received by the television receiver.